Nature of Fast Relaxation Processes and Spectroscopy of a Membrane-Active Peptide Modified with Fluorescent Amino Acid Exhibiting Excited State Intramolecular Proton Transfer and Efficient Stimulated Emission

A fluorescently labeled peptide that exhibited fast excited state intramolecular proton transfer (ESIPT) was synthesized, and the nature of its electronic properties was comprehensively investigated, including linear photophysical and photochemical characterization, specific relaxation processes in the excited state, and its stimulated emission ability. The steady-state absorption, fluorescence, and excitation anisotropy spectra, along with fluorescence lifetimes and emission quantum yields, were obtained in liquid media and analyzed based on density functional theory quantum-chemical calculations. The nature of ESIPT processes of the peptide’s chromophore moiety was explored using a femtosecond transient absorption pump-probe technique, revealing relatively fast ESIPT velocity (∼10 ps) in protic MeOH at room temperature. Efficient superluminescence properties of the peptide were realized upon femtosecond excitation in the main long-wavelength absorption band with a corresponding threshold of the pump pulse energy of ∼1.5 μJ. Quantum-chemical analysis of the electronic structure of the peptide was performed using the density functional theory/time-dependent density functional theory level of theory, affording good agreement with experimental data

Electronic Nature Of Nonlinear Optical Properties Of A Symmetrical Two-Photon Absorbing Fluorene Derivative: Experimental Study And Theoretical Modeling

A comprehensive experimental and theoretical study of linear photophysical properties, such as excited-state relaxation, two-photon absorption, and stimulated emission spectra of the symmetrical fluorene derivative 2,2′-((1E,1′E)-(9,9-diethyl-9H-fluorene-2,7-diyl)bis(ethene-2,1-diyl))bis(1-methyl-1H-pyrrole) (1), is presented. The steady-state absorption, fluorescence, excitation, and excitation anisotropy spectra of 1 in organic solvents of different polarities are investigated experimentally and modeled. The fluorescence solvatochromism of 1 suggests the occurrence of symmetry breaking in the first excited state. The nature of fast relaxation processes in the excited state of 1, with the characteristic times of several picoseconds, is investigated by transient absorption femtosecond pump-probe spectroscopy. The spectral properties of 1 are satisfactorily described by an essential-state model that, accounting for electron-vibration coupling and for polar solvation, addresses spectroscopic features not only in terms of band position and intensities but also in terms of band shapes. Specifically, we present the first calculation of frequency-resolved two-photon-excited fluorescence anisotropy spectra. Our results demonstrate that electron-vibration coupling and polar solvation quite naturally explain the puzzling experimental observation of large deviations of the anisotropies from the values expected on the basis of the relative orientation of the molecular transition dipole moments